Earth Science Chapter 14 Vocabulary and Facts Tools A B magma molten rock underground volcano magma that reaches the surface and erupts through an opening rate of volcano determined by silica content felsic magmas high silica content are thick, light-colored and slow moving mafic relatively low silica content - thinner and darder in color - flow more easily magma rises from the asthenosphere through cracks in the lithosphere most important gases in magma water vapor, carbon dioxide, and sulfur other volcanic gases hydrogen, chlorine, and fluorine Combinations of important gases in magma include sulfide, and sulfur dioxide the kind of eruption is determined by the amount of gases dissolved in a magma as magma reaches the surface pressure on it is greatly reduced dissolved gases come out of solution as bubbles - they can even explode magmas with large amounts of dissolved gases tend to produce more explosive eruptions lava magma that reaches the surface lavas can be felsic (thick & stiff) or mafic (thin & fluid) mafic magmas allow gases to dissolve withing them escape easily felsic magmas allow gases to cause a more exploseive eruption tephra solid fragments of lava smallest tephra ash lapilli larger pieces of tephra (up to 64 millimeters) blocks largest fragments (> 64 millimeters) that erupt as solid pieces bombs largest fragments (> 64 millimeters) that are liquid and harden as they fall dense,superheated cloud that travels downhill and are very dangerous a combination of tephra and gases rift eruptions occur a long, narrow fractures in the crust eruptions flow out smoothly and fluidly in a rift eruption because the lava is basaltic and contains few gases shield cone a volcanic mountain with a broad base and gently sloping sides occur at spreading centers (Mid-Atlantic Ridge and E. Pacific Rise) lava oozes out into rounded shapes called pillow lavas rift eruptions on land may spread lava evenly over thousands of sq. kilometers lavas from the E. African Rift system formed a basalt plateau examples of a basalt plateau Columbia Plateau of Washington, Oregon, and Idaho Karroo Plateau of S. Africa and Parana Plateau of S. America basalt plateau columnar jointing a unique pattern of closely packed, six-sided columns caused by cooled thick lava subduction boundary eruptions magma - thick and contain large amounts of gases explosive eruptions are subduction boundary eruptions cinder cone volcanic cone that forms from subduction eruptions - steep sides Example of volcanic chains Philippine Islands, islands of Japan, & Aleutian Islands (Alaska) hot spots areas of vocanic activity in the middle of lithospheric plates hot spot lavas form cones Example of hot spot volcanism Hawaiian Islands Volcano warning signs changes in volcano's slope, bulges, increase in small earthquakes Eldfell volcanic mountain off of iceland, near mid-Atlantic rift-> rift eruption Mt. St. Helens Washington, one of 15 in Cascade range - subduction boundary volcanism Kilauea Hawaii,shield volcano resulted from hot spot Vesuvius Mediterranean, subduction volcano Krakatau Indonesian chain, subduction volcano - most violent eruption in history Crater Lake Oregon, caldera (huge crater) formed when cone collapsed-subduction Mount Pinatubo 20th Cent. biggest vocanic eruption - subduction Olympus Monds largest volcanic cone in the solar system - Mars Io a Jupiter moon that has 100 active/inactive volcanoes. plutons or igneous intrusions rock masses that form when magma cools inside other rocks dikes sheets of igneous rock that cut across rock layers they intrude. sills sheets of igneous rock that are parallel to the layers they intrude laccoliths domelike masses that bulge upward batholiths largest igneous intrusions, exposed by erosion - granite or grandiorite stock small batholith less than 100 sq. km, exposed at the surface